The invention is related to a sine wave generator composed of digital circuits.
More specifically, the invention is related to a sine wave generator designed for generating sine waves of different frequencies one of which frequencies can be selectively chosen at a specific time.
Besides completely analog sine wave generators of conventional design, there are known digital circuit arrangements for generating a simulated sine or cosine wave form which arrangements are often used in connection with measurement devices. These circuit arrangements are based upon algorithms providing a mathematical approximation of sine waves as, for example, in the form of ##EQU2## where C.sub.1, C.sub.3, C.sub.5 are constants. Such an algorithm can be implemented in a computer system which calculates sine values for generating a sine or cosine wave form.
A somewhat different approach, not to calculate but to store respective sine or cosine values is known from a sine-cosine-generator described in U.S. Pat. No. 3,569,684 to Burnett. The known generator is composed of a diode array forming a fixed memory for storing binary values of the sines of angles and of means for addressing memory locations which means are integral with the memory. Such a circuit arrangement can be very useful for applications, in which analog values of angles measured in radians are picked off, for example, from a gyro or other inertial instruments and converted into a binary number representing the angle. This number is used as an input to the address means of the system for selecting the location in the fixed memory wherein the sine of the angle is stored. Information is read out and compared with the value of the sine of the angle which should have been read from the memory of a computer. If the actual value is different from the required value, an error signal is generated to correct the position of the instrument from which the angle was picked off.
Another digital-to-analog converter for converting digital angular data to analog angular data for controlling servo mechanisms and the like has been described in U.S. Pat. No. 3,728,719 to Fish disclosing an R-2R-resistive ladder network of a digital-to-analog converter. This converter has inputs for digital angular data which are applied to integrated switches to cause predetermined switching-on of resistors of the ladder network, thus providing sine and cosine approximation of analog voltage functions representing the digital input signals. Some of the digital signals control quadrant switches to produce quadrant reference phase relationships and the remainder of such control analog signals provide amplitude scaling to drive a synchromechanism in angular correspondence with the digital angular data input circuit. The ladder network consisting of R-2R-stages together with correspondingly controlled switches supplies a sine or cosine function. The practical instrumentation simulates an approximation of a sine wave form. This approximation becomes more and more accurate with a higher number of resistor stages. In principle, the known circuit arrangement is based upon the assumption that any analog value can be composed of digital values which are represented by respective resistor stages and the associated switch in the known circuit arrangement. A systematic error, therefore, is due to the finite combination of ladder stages.
A similar circuit arrangement is known from U.S. Pat. No. 3,974,498 to Knier disclosing a switching arrangement for the transformation of digital angles into analog sine- and/or cosine values. A resistance network is provided consisting of a series of individual resistors whose resistance values are selected according to a certain function. A digital control register buffers the input angle in digital form and has a respective series of resistor stages for representing digital values which are graded so that the control register is capable of representing angles at least in a range of 0.degree. to 45.degree. according to the same function as the resistance values and with a respective one of the register stages thus being assigned to each of the individual resistors. There is also provided a constant voltage source such that a reference voltage supplied by such source will be essentially independent of the angle value registered in the digital control register. The converter circuitry further includes a selectively switched correcting circuit, preferably in the form of a second resistance network and a series of switches controlled in accordance with the input angle to create an angle-dependent compensating function whose analog variation as a function of the input angle modifies the overall analog output of the converter circuitry so as to approximate the exact course of the trigonometric function or functions to a desired precision.
Basically, all those known digital-to-analog converters generate an analog output signal corresponding to a digital input signal, both signals representing a specific value such as an angle value which value is supplied to the digital-to-analog converter in digital form to control the operation of the converter. All these circuit arrangements are of relatively complex design. This design reflects the preferred applications of the circuit arrangements described above as parts of measurement devices where high precision is required.
In a variety of applications for sine wave generators, however, the precise angular approximation of the sine wave is less important than is a highly stabilized frequency. Representing sine wave generators of this type, a digital-to-analog converter designed as function generator is known from "Advanced Electronic Circuits" of Tietze and Schenk, edited by Springer-Verlag New York, 1978, Chapter 14 "D/A and A/D Converters," pages 411 through 441. Especially with reference to FIGS. 14.18 through 14.21 there is described an implementation of a digital-to-analog converter which is used as sine wave function generator. The generator employs two resistor networks. Each of which networks is composed of a variety of rated resistors which are commonly connected to either a positive or a negative supply voltage, respectively. Each resistor of both networks is selectively connectable to a common output line via a respective switch. The signals carried on the output line are supplied to an inverting input of an operational amplifier designed as comparator having a positive input which is coupled to ground.
The rating of the resistors is such a piece-wise approximation of a sine wave is achieved if the switches are selectively operated in a given sequence with equidistant time intervals. The switches can be implemented by an analog multiplexer circuit which is controlled by a cyclic straight binary counter. This counter, in turn, receives a pulse train of predetermined frequency. Any change of the frequency of this pulse train results in analog output signal train of different frequency since input frequency and output frequency are directly related to each other.
Another characteristic of this known circuit arrangement is that just one of the resistors of both networks is connected to the common output at a time and that a current determined by this operable resistor and the associated direct current voltage source defines the present step value of the resulting sine wave. Compared with the previously described devices, the outlay of this known device is rather small and the achieved performance accordingly also relatively unprecise. The main restriction of the known device is based upon the scheme of the selective switching arrangement.
It is, therefore, an object of the present invention to provide an improved sine wave generator which is composed of digital circuits.
Another object of the present invention is to provide such a sine wave generator which includes a highly efficient network for simulating a sine wave shape while still being easily adjustable for generating sine waves of different frequencies.